You don't. As the sourcebooks point that out, people of any belief system can be Haqq citizens, though certainly People of the Book (i.e. followers of the Abrahamic religions) are more welcome than others. The difference Haqqislamites and non-Haqqislamites (in terms of religion) are mostly in tax regulations: non-Haqqislamites pay a different taxthat is, if memory serves me, used to fund social services. I don't think we were told how high which tax is, but it has to be witihn acceptable margins. Actually, it reminds me of the current German system: you either declare yourself as a follower of a religion of choice and pay "church tax" that goes to the upkeep of the given religion (temples, priests, etc are being paid by the state from this tax), or as non-believer (and you pay another tax spent for charity). Actually in Germany the "charity tax" is somewhat lower than "church tax"... :P
@Errhile If anything, I think it's a reference to general policies during golden age of Islam. People of the Book were considered worthy of dealing with, although sometimes they paid extra compared to converted populace (for example, when tributes were concerned). Extra "social tax" of Haqqislam reminds me of Zakah (or Zakat?), which is one of the five "Pillars of Islam" and is a tax that is supposed to be used exactly for that. The difference is that Zakah is paid by everyone, but I guess CB just fused several concepts/references together to create sufficiently modern-looking system for the futuristic universe in question.
I think that in Haqqislam, if you are of the Haqqislamite faith you receive a tax based on that, which then goes to certain things. If you aren't, then you pay a different (and, one would imagine, very similar) tax which then goes to other things. I think that the Haqqislamite tax is used to fund things like the state's religious initiatives, which other people aren't required to contribute to as they don't take part in that.
Zakat ("charity") is indeed paid by every Muslim (able to do so, and thus not entitled to receive zakat). However, there was a practice of levying another tax on non-Muslims (who were, by the facto f being non-Muslim, exempt from zakat, as well as not expected to follow the other Pillars of Islam - shahada, the Faith, hajj, the Pilgrimage, salat, the prayer, and sawm, fasting), which was then called jizya. Sure, CB had cooked it up, but I think there's enough historical examples of similar traditions to make it beliveable.
